Semiconductor devices are manufactured by depositing many different types of material layers over a semiconductor workpiece or wafer, and patterning the various material layers using lithography. The material layers typically comprise thin films of conductive, semiconductive, and insulating materials that are patterned and etched to form integrated circuits (IC's).
For many years in the semiconductor industry, optical lithography techniques such as contact printing, proximity printing, and projection printing have been used to pattern material layers of integrated circuits. Projection printing is commonly used in the semiconductor industry using wavelengths of 248 nm or 193 nm, as examples. At such wavelengths, lens projection systems and transmission lithography masks are used for patterning, wherein light is passed through the lithography mask to impinge upon a wafer.
However, as the minimum feature sizes of IC's are decreased, the semiconductor industry is exploring the use of alternatives to traditional optical lithography techniques, in order to meet the demand for decreased feature sizes in the industry. For example, short wavelength lithography techniques, Scalpel, other non-optical lithographic techniques, and immersion lithography are under development as replacements for traditional optical lithography techniques.
One lithography technique under development is immersion lithography, in which the gap between the last lens or element in the optics system and a semiconductor wafer is filled with a liquid, such as water, to enhance system performance. The presence of the liquid enables the index of refraction in the image space, and therefore the numerical aperture of the projection system, to be greater than unity. Thus, immersion lithography has the potential to extend 193 nm tools used in lithography down to about 45 nm or below, for example.
FIG. 1 shows a perspective view of portion of a prior art immersion lithography system. The prior art immersion lithography system is described in “IC Knowledge Technology Backgrounder: Immersion Lithography”, from the website: http://www.icknowledge.com/misc_technology/Immersion%20Lithography.pdf, which is incorporated herein by reference. An immersion lithography system is described in further detail in U.S. patent application Ser. No. 2005/0046813 A1, published on Mar. 3, 2005, which is also incorporated herein by reference.
The portion of the immersion lithography system 100 shown in FIG. 1 includes a wafer 102 mounted on a wafer support 104. The wafer support 104 is also referred to as a wafer stage or exposure chuck, for example. A projection lens system 108 is disposed proximate the wafer 102. A fluid 106 such as water is disposed between the last element 110 of the lens system 108 during the lithography process, e.g., by an immersion head clamped to the end of the lens system 108 (not shown in FIG. 1: see FIG. 2 at 120). A stepper or scanner (not shown) moves the stage or wafer support 104 during the patterning of the individual die or regions of die 112 on the wafer 102. The fluid 106 is typically provided by a nozzle or by input and output ports within the immersion head 120 (see FIG. 2), for example.
FIG. 2 shows a more detailed cross-sectional view of the portion of the prior art immersion lithography system 100 shown in FIG. 1. The immersion lithography system 100 includes an immersion head 120 disposed proximate the last element 110 of the lens system 108. The immersion head 120 includes ports 122 and 124 for supplying the fluid 106 between the wafer 102 and the immersion head 120. The ports 122 and 124 may comprise input and output ports, for example. Hoses (not shown) may be coupled to the ports 122 and 124 for injecting H2O or other fluids, for example. The immersion head 120 typically includes a bottom plate 128 that is transparent, as shown. The bottom plate 128 keeps the liquid 106 from reaching the last element 110 of the lens system 108 and also prevents gases that may be outgassed from a photoresist 116 on the wafer 102 from reaching the last element 110, for example. The immersion head 120 may also include vacuum ports 126 disposed proximate the fluid ports 122 and 124. The vacuum ports 126 may be used to ensure that the fluid stays only immediately beneath the immersion head 120 central region, for example. An immersion head 120 such as the one shown in FIG. 2 is also referred to in the art as a shower head, for example.
The wafer 102 typically includes a workpiece 114 with a layer of radiation sensitive material 116 such as photoresist disposed thereon. The pattern from a mask or reticle (not shown) is imaged onto the photoresist 116 using a beam 118 of radiation or light emitted from the lens system 108. After exposure of the photoresist 116, the patterned photoresist 116 is later used as a mask while portions of a material layer (not shown) disposed over the workpiece 114 are etched away (also not shown).
FIG. 3 illustrates a prior art closing disk 130 that makes contact with a bottom surface 132 of an immersion head 120. The closing disk 130 is typically used to cover the immersion head 120 while a wafer 102 is being removed from the wafer support 104 (not shown in FIG. 3: see FIG. 1) and replaced with the next wafer 102 to be processed, for example. In an immersion lithography system 100, it is important to keep the immersion head 120 wet and to avoid drying of the fluid 106, in order to avoid forming drying stains from the fluid 106 on the bottom plate 128. Drying stains from the fluid 106 would reduce the intensity of the illumination energy that reaches the wafer 102 (see FIG. 2) and over time would diminish the resolution of the exposure tool or immersion lithography system 100, for example. Thus, the closing disk 130 is used to ensure that the bottom plate 128 of the immersion head 120 remains wet during wafer 102 exchanges, for example.
The closing disk 130 allows the wafer stage and exposure chuck 104 (see FIG. 1) to move from under the lens system 108 for removing an exposed wafer 102 from the exposure chuck 104 and loading of a new wafer 102. The closing disk 130 may be kept on the same wafer support 104 or may reside elsewhere proximate the immersion head 120, for example. The closing disk 130 is moved under the immersion head 120, and the immersion head 120 may lift the closing disk 130 using the vacuum ports 126, for example, or alternatively, the immersion head 120 may be placed in contact with the closing disk 130 while it remains positioned on a wafer support 104 (as shown in FIG. 1), for example.
A problem with the prior art closing disk 130 shown is that because direct contact is made to the entire bottom surface of the immersion head 120, the closing disk 130 can shift and scratch the immersion head 120 bottom surface 132. The top surface of the closing disk 130 can also become scratched during the contact with the immersion head 120 bottom surface 132, for example. This can create particulates, e.g., debris from the scratched immersion head 120 and/or closing disk 130, and the particulates can enter the fluid 106. The particulates can adhere to the immersion head 120 and can also be deposited onto the wafers 120 during exposure, resulting in decreased device yields, for example.
Furthermore, when the closing disk 130 is removed, the fluid 106 flows under the immersion head 120, because the fluid 106 typically continues to flow during the removal of the closing disk 130. Because the closing disk 130 comes into direct contact with the bottom surface 132 of the immersion head 120, no or little fluid 106 is present in those areas, creating thermal instability. As the immersion fluid 106 and the immersion head 120 are not temperature controlled, there is a period of time wherein the immersion fluid 106 is not thermally stable, when the closing disk 130 remains in place on the immersion head 120, as an example.
Thus, what are needed in the art are improved designs for closing disks of immersion lithography systems.